How to tell if the Tesla home battery will save you money

Attendees take pictures of
the new Tesla Energy Powerwall Home Battery during an event at
Tesla Motors in Hawthorne, California April 30, 2015. Tesla
Motors Inc unveiled Tesla Energy - a suite of batteries for
homes, businesses and utilities - a highly-anticipated plan to
expand its business beyond electric vehicles.REUTERS/Patrick T. Fallon

Tesla's Powerwall home batteries
comprise a high-profile entry by the electric-car maker into a
new segment of the energy industry that's expected to grow
rapidly.

Energy storage potentially allows homes and
businesses to make greater use of renewable sources like solar
and wind, and gives them a degree of flexibility in power
rates.

For home users, the main benefit of the
Powerwall battery is its ability to store excess energy generated
by an existing solar-panel installation for later use.

While that lowers the energy use and carbon
footprint of individual homes, could it also save owners
money?

North American residents can now explore that
question, thanks to a Powerwall cost
calculator created by Bruce Lin and available on
his Catalytic Engineering blog.

The calculator assumes a home with enough
solar-panel surface area to generate 7 kilowatt hours of surplus
energy 365 days per year.

That's enough to fill the smaller of the two
available Powerwall battery packs, which sells for $3,000 before
installation. Tesla will also sell a 10-kWh pack for
$3,500.

The baseline rates are not those that prevail in
the U.S., however, but those in Germany: $0.34/kWh for
electricity purchased from the utility, and $0.12/kWh for
electricity sold back to a utility company.

Owners then enter values for their own
situations into the calculator, including electricity rates,
installation costs, and other factors.

For the test case with German baseline values,
though, Lin concluded that buying a Powerwall virtually breaks
even with simply selling excess electricity generated by an
existing solar array back into the grid.

The baseline rate for home electricity purchased
from a utility in the U.S., however, is roughly one-third the
cost in Germany: It average $0.12, rather than $0.34.

Using that rate, the Powerwall isn't a way for
most home users in the U.S. to save money today--although Hawaii,
with an average rate of $0.37 per kWh, is an exception.

As battery costs come down and solar/battery
installers begin competing for business, Lin believes a Powerwall
battery pack could become more attractive to larger numbers
of homeowners on a
purely economic basis.

And in Germany, it may already be economically
advantageous to buy a Powerwall system--assuming installation
costs are kept in check, the house has a fairly large solar
array, and is in a sunny part of the country.

Tesla CEO Elon Musk has mentioned both
Germany and Australia as markets where a purely economic case
could be made for the Powerwall today.

However, it's likely the majority of early
Powerwall purchasers in the U.S. will be enticed more by the
environmental benefits than by any potential cost savings.

That's particularly the case for electric-car
owners, who can lower the overall carbon emissions associated
with their vehicles by
recharging them from renewable sources.